U.S. Pat. No. 5,670,115 to Cheng et al. discloses a thin film of palladium to catalyze the dissociation of Hydrogen gas. Detection and measurement of the hydrogen gas is accomplished with an amorphous metal film consisting of nickel and zirconium that has a resistance which varies with the concentration of dissolved hydrogen. This device consists of only two layers and operates in the range of room temperature to 150° C. According to Cheng, palladium film serves to dissociate hydrogen molecules at the Pd surface and the hydrogen atoms diffuse into the palladium film. Hydrogen atoms diffuse through the thin palladium film into the underlying nickel-zirconium film and dissolve therein. Hydrogen atoms flow into and out of the films depending on the hydrogen gas concentration. The electrical resistivity of the nickel-zirconium film increases as the content of the dissolved hydrogen increases. The palladium layer also serves as a barrier to oxidation of the underlying nickel-zirconium film. See, the '115 patent to Cheng at col. 2, lns. 1 et seq.
U.S. Pat. No. 3,709,810 to Grubb et al. discloses the use of an improved hydrogen ion selective sensing electrode comprising a palladium oxide coated surface on a palladium coated base member along with a reference electrode in contact with an electrolyte chamber.
U.S. Pat. No. 6,184,564 to Gould discloses a Schottky diode having a palladium silicide on silicon barrier in which the barrier height is adjusted by adding a small quantity of another metal during the deposition of the palladium. The palladium silicide includes palladium and a small quantity of another metal. The additional metal is chosen depending on whether barrier height is raised or lowered. See col. 1, lns. 59 et seq of the '564 patent to Gould. Gould does not mention hydrogen detection.
U.S. Pat. No. 5,783,153 to Logothetis et al. discloses a sensor made from a metal or its oxide which is capable of changing from one metal or metal oxide phase to another. The oxygen sensitive material disclosed is palladium which changes phase to palladium oxide when the partial pressure of oxygen reaches a certain value. This phase change causes a change in the material's conductivity which can be measured.
U.S. Pat. No. 6,818,523 to Miki et al. teaches the method for producing a semiconductor storage device comprising a hydrogen diffusion preventing layer. The semiconductor storage device comprises a capacitor electrode with a film to reduce the amount of hydrogen reaching the capacitor electrode. Miki discloses the use of palladium oxide as a hydrogen diffusion preventing layer. See FIG. 3 wherein Miki et al. discloses a diffusion layer 302. Miki at col. 3 lns. 24 et seq. and at col. 7, lns. 45 et seq. identifies palladium oxide as preventing hydrogen diffusion. Miki et al. uses capacitors comprising stacked layers and palladium oxide as a conductor and not as a dielectric.
U.S. Pat. No. 6,730,270 to O'Connor discloses a single-chip hydrogen sensor wherein a silicon-based hydrogen sensor portion is comprised of a first material and an interconnect metallization layer of the same material. The first material taught in the application is a palladium nickel alloy. The interconnect metallization is covered with an oxide or a nitride to make the interconnect metallization inert. See col. 2 lns. 21-36.
U.S. Pat. No. 6,109,094 to Baranzahi et al. teaches the use of a gas sensing device having a semiconductor substrate wherein the semiconductor substrate is covered by an insulator layer on which an intermediate layer is formed, and is subsequently covered by a gas sensing catalytic layer. The semiconductor substrate disclosed in this patent is silicon carbide or diamond. The intermediate layer is a silicide. The device can be operated at 600° C. continuously. Baranzahi et al. indicates at col. 4 lns 27 et seq. that the voltage-capacitance curve shifts for a MOSiC device.
FIG. 1 illustrates a Pd/SiO2 hydrogen sensor 100 illustrated in an article entitled “Hydrogen Sensing Mechanisms Of Metal-Insulator Interfaces,” Acc. Chem. Res. 1998, 31, pp. 249-256, by LARS-GUNNAR EKEDAHL, MATS ERIKSSON, and INGEMAR LUNDSTROM. FIG. 1 illustrates a catalytic metal Pd which dissociates molecular hydrogen into atomic or elemental H+ which then forms a dipole layer at the interface of the Pd 103 and the SiO2 104. SiO2 is an insulator and a Si substrate 105 is used and is interconnected to ground 102. A voltage 101 is supplied across the device which functions as a capacitor.
FIG. 1A illustrates 100A a shifting of the voltage-capacitance curve as a function of the hydrogen content. As the hydrogen content increases the magnitude of the CV shift or ΔV increases in proportion to the charge concentration and separation (for example, the dipole moment) as set forth on page 250 of the reference cited immediately hereinabove. Reference numeral 106 represents the CV curve without the presence of hydrogen and reference numeral 107 represents the CV curve with the presence of hydrogen.
FIG. 1B illustrates 100B the dipole layer 104A formed at the interface of the Pd and the insulator SiO2. Hydrogen (H2) 109 is dissociated into atomic hydrogen (H+) and the magnitude of the dipole layer is dependent on the amount of hydrogen available.
An article entitled “PHYSICS WITH CATALYTIC METAL GATE CHEMICAL SENSORS”, VOL. 15, Issue 3 (1989) by Ingemar Lundstrom, Marten Armgarth, and Lars-Gunnar Petersson, also discloses the palladium-silicon dioxide-silicon capacitor and other structures.
U.S. Pat. No. 6,265,222 to DiMeo, Jr. et al. teaches the use of a hydrogen sensor including a hydrogen-interactive metal film that reversibly interacts with hydrogen to exhibit a detectable change. A thin film hydrogen permeable barrier layer is used to protect the hydrogen interactive layer from deleterious interaction with non-hydrogen species. DiMeo discloses the use of palladium as the thin film permeable barrier and rare earth metals as the hydrogen interactive layer.
In an article authored by Frank DiMeo Jr., PhD., entitled “Integrated Micro-Machined Hydrogen Gas Sensor” prepared for and sponsored by the United States Department of Energy, DOE/GO/10451-F, a recountal of existing hydrogen sensing technology is found. The article discloses (at page 5 thereof) a gated field effect type transistor like structure having a floating gate that is coated with a catalyst, typically palladium. As the palladium gate adsorbs hydrogen the potential of the gate changes and modulates the conductance of the channel. Dr. Dimeo goes on to indicate that the device is quite sensitive but tends to saturate at low levels of hydrogen making it unsuitable for explosive limit detection. The article goes on to discuss another hydrogen sensor which is based on resistivity changes that occur as a function of hydrogen content in palladium or palladium alloys. Dr. DiMeo does not indicate the structure of these devices and claims that they were not sensitive.
The article entitled “Development of SiC-based Gas Sensors for Aerospace Applications”, Mat. Res. Soc. Symp. Proc. Vol. 815, 2004 by G. W. Hunter et al. discloses the use of chrome carbide as a barrier layer between the catalytic metal and the SiC semiconductor. Although this composition showed no indication of massive silicide formation between the catalytic sensing layer and the semiconductor substrate, this sensor showed limited sensitivity to hydrogen and showed signs of chromium migration to the surface as well as formation of chromium oxide.
Use of catalytically active resistors is based on the concept that hydrogen migrates into the resistor and changes the resistance of the sensor. Palladium and its alloys are common resistor materials. The use of palladium as the hydrogen sensitive metal is problematic because a phase change occurs at high hydrogen concentrations. Use of palladium, however, at low hydrogen concentrations does not cause a phase change to occur and Pd alloys can be used for higher hydrogen concentration measurements. See the article entitled “The Development of Hydrogen Sensor Technology at NASA Lewis Research Center, NASA”-Technical Memorandum-106141 (1992) by G. W. Hunter et al.
There is a growing demand for high temperature gas sensors with high sensitivity for engine emission monitoring, fire detection, and fuel leak detection. In particular, high temperature gas sensors with high sensitivity are of interest for aerospace applications including: monitoring emissions from high temperature combustion systems or chemical processing applications, monitoring of fuel leaks in launch vehicles, and fire detection on-board commercial and space vehicles.
A Schottky diode sensing structure can be used to measure hydrogen concentration due to its high gas sensitivity. A Schottky diode can be generally defined as a metal in contact with a semiconductor (MS) or a metal in contact with a thin insulator (MIS) or oxide (MOS) on a semiconductor.
Hydrogen (H2) dissociates on the surface of the metal leading to the formation of a dipole layer at the interface of the metal and the semiconductor lower layer. The dipole layer leads to a change in the forward or reverse current and a change in the capacitance. The height of the potential barrier is a function of the materials used and their temperature. See the article entitled “Development of SiC Gas Sensor Systems,” NASA/TM-2002-211707 by Hunter et al. The barrier height depends on the work function of the metal and the electron affinity of the semiconductor. Further, use of a Schottky diode allows hydrogen to be detected without requiring high voltage. A small change in the concentration of hydrogen can be reliably detected.
The article entitled “Development of SiC Gas Sensor Systems”, NASA/TM-2002-211707, by G. W. Hunter et al. discloses two structures to improve the stability of the palladium based Schottky diode structures over that of the Pd/SiC Schottky sensor. The first structure includes the incorporation of chemically reactive oxides such as SnO2 (tin oxide) for a MOS device. SiC devices can be operated at high temperature to be reactive to hydrocarbons resulting in a MROS (metal reactive oxide semiconductor). Tin oxide (SnO2) is recited in the article as a reactive oxide. The second structure is PdCr/SiC which has shown good response and stability for some samples but for others has the drawback of silicide formation at the interface of the metal and the semiconductor. The article also mentions a wide variety of materials sensitive to hydrocarbons that may be used without specifying them.
The temperature range for hydrogen detection as is identified hereinabove is beyond the upper limit for substrates made from Silicon-based semiconductor substrates. The use of silicon carbide allows for hydrogen detection in the demanding range of conditions required in aerospace applications. Although silicon carbide has excellent high temperature performance, the sensitivity of the device is limited by the reliability and stability of the interfaces between the silicon carbide semiconductor substrate layer and the other layers of the device. At high temperatures, reactions between the various different layers can lead to the formation of silicide materials. These reactions lead to reduced sensitivity and disruptions of the device. The reaction between the layers is a problem for high temperature application requirements where it is difficult to optimize both sensitivity and stability of the device.
In addition to diodes, other devices including capacitors, Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETS), Metal-Semiconductor Field Effect Transistor (MESFET), and Metal-Insulator-Semiconductor Field Effect Transistor (MISFET) are used as gas sensors. Catalytic metals are used as gates for gas sensitive field effect devices. In addition to the gate, these devices typically contain electrodes, a source and a drain, interconnected by a channel region. The channel carries current between the source and the drain. Varying potential of the gate affects the amount of current flowing in a MESFET structure. In MISFET devices, a gate insulator material is located between the gate electrode and the channel.
Generally, a metal oxide is located between the gate electrode and the channel in a MOSFET structure. Use of a catalytic sensing metal, for example palladium, in the gate allows hydrogen detection to occur when the hydrogen gas is disassociated into atomic hydrogen and adsorbed onto and into the catalytic sensing metal causing a change in the electronic properties of the device.
A description of electronic semiconductor based gas sensors including MOS (metal oxide semiconductors), MIS (metal insulator semiconductors) and MRIS (metal reactive insulator semiconductor) technology can be found in articles entitled “Chemical Gas Sensors for Aeronautics and Space Applications,” NASA Technical Memorandum 107444, May 1997 and in “Chemical Gas Sensors for Aeronautics and Space Applications II,” NASA Technical Memorandum, 1998-208504, 1998.
At high temperatures many gas detecting sensors are not able to maintain sensitivity and stability due to chemical reactions occurring between the catalytic metal sensing layer and substrate layer or between the catalytic sensing layer, barrier layer, and substrate layer. Typically, reactions between the layers of the gas detecting device can lead to the formation of metal silicides on the interface between the metal or metal alloy layer and the substrate layer. The silicide materials which may form render the overall sensor insensitive to hydrogen and hydrocarbon materials. As a result, formation of silicide materials leads to decreasing hydrogen and hydrocarbon detection sensitivity, undesired oxidation, disruption, and degradation of the sensor device. Silicides are understood to incapacitate the sensor. At the same time, to be an effective hydrogen gas detector it is important for the sensor to be able to function at temperatures as high as 600° C. without interruption.
A further problem found in the prior art (Logothetis et al., U.S. Pat. No. 5,783,153) is hysteresis which contributes to the decreasing accuracy of the device at high temperatures. It is known that some metals react with oxygen resulting in the formation of a metal oxide. However, one form of metal oxide can change to another metal oxide phase when the temperature or atmosphere is changed. The different oxidation states of a metal or metal alloy also can result in changes in the resistivity of the metal or metal alloys. Hysteresis is related to phase changes between states brought on by chemical reactions leading to nonreversible changes. Hysteresis can lead to longer response times in the sensor and damage to the metal film used in the device. Phase changes can lead to problems which can affect both the sensitivity and stability of the sensor.